The invention relates to a catalytically active structure, in particular a honeycomb structure, through which an exhaust gas from a combustion engine can flow and which is formed by at least one metal sheet which has a catalytically active surface.
In order to reduce polluting components, for example unburnt hydrocarbons, carbon monoxide and nitrogen oxides, structures, in particular honeycomb structures, are used which have a catalytically active surface. The structure may be formed by a monolithic ceramic honeycomb body. Structures are also known which are formed by rolled up and/or stacked metal sheets. The metallic structures are coated with a thin layer of aluminum oxide (wash coat). The aluminum oxide increases the surface area of the structure. A catalytically active material is distributed over the aluminum oxide layer. The material contains platinum, rhodium and palladium.
In order to optimize the way in which the exhaust-gas catalytic converter functions, the air-fuel mixture in the exhaust gas needs to be controlled accurately. To this end, a lambda probe is arranged in the exhaust pipe, upstream of the exhaust-gas catalytic converter relative to the flow direction of the exhaust gas. The lambda probe is connected to a control system of the combustion engine, through the use of which a formation of the mixture of the combustion engine can be controlled. Such systems for reducing the polluting components of an exhaust gas are also known as three-way catalysts for motor vehicles.
According to Nonnenmann in his article xe2x80x9cMetalltrager fur Abgaskatalysatoren in Kraftfahrzeugenxe2x80x9d [Metal supports for exhaust-gas catalytic converters in motor vehicles] MTZ Motortechnische Zeitschrift, volume 45 No. 12/1984, there have also existed metal catalysts made of catalytically active metal sheets, for example monel, but which were not successful.
Also in the field of small motors, for example motors for lawnmowers, chainsaws, motor cycles with small cylinder capacity and the like, it is desirable to reduce exhaust-gas components from such combustion engines using catalytically active structures. However, such combustion engines do not have a motor control system, and do not have lambda probes in the exhaust pipe, with the result that it is not possible to control the formation of the mixture in order to minimize pollution.
Small motors run on a rich mixture, that is to say with an excess of fuel. With such combustion engines, the problem arises that a large amount of unburnt hydrocarbons reach a catalytically active structure and are combusted there. Hence, the risk arises of so-called hot spot formation, which can destroy the catalytically active surface. In particular, there is a risk that, when a catalytically active structure which has a wash coat is used, the wash coat may become vitrified so that the catalytically active structure can achieve only a short working life.
It is accordingly an object of the present invention to provide a catalytically active structure that overcomes the above mentioned disadvantages of the prior art devices and methods of this general type and which is also suitable for a combustion engine that does not have a system for controlling the formation of the mixture.
With the foregoing and other objects in view there is provided, in accordance with the invention, a catalytically active honeycomb structure through which an exhaust gas from a combustion engine can flow, comprising at least one metal sheet having a catalytically active surface being at least partially provided with a catalytically active metal oxide layer, wherein the metal oxide is an oxide of a metal from the fourth period selected from the group consisting of Ti, V, Zn, Fe, Cr, Mn, Ni, Cu and Co.
The specific activity of the metal oxide layer is in this case lower than the specific activity of the catalytically active metals selected from the noble metal group, as are used in three-way catalysts. Such a catalytically active structure is, in particular, suitable for combustion engines which do not have a system for controlling the formation of the mixture since only a part of the surplus fuel is catalytically converted at the surface of the metal oxide layer and so-called hot spot formation can therefore be avoided.
The metal oxide layer is formed directly on the metal sheet, so that it is possible to do without application of a support layer in the form of a wash coat, which simplifies the production of such a structure.
According to an advantageous refinement of the structure according to the invention, the metal of the metal oxide layer can be a constituent of the material of the metal sheet. The advantage of this embodiment is that the metal of the metal oxide layer is integrated in the material of the metal sheet, which provides good bonding of the metal oxide layer to the metal sheet. The metal oxide layer can be formed by heat-treating the metal sheet in an oxidizing atmosphere.
According to a further advantageous development of the structure, the metal oxide layer can be formed by oxidation of a metal layer applied to the metal sheet. The metal layer can, for example, be applied by rolling onto the metal plate. The metal layer can also be applied to the metal sheet by dipping the base material in a liquid metal. As an alternative, the metal layer can be applied to the metal sheet by spraying. In this case, the metal forming the metal layer can be sprayed in liquid form or as dust. Depending on which metal forms the metal layer, the metal layer can also be formed by electrolytic deposition.
It is not absolutely necessary for the metal layer to be applied over the entire surface of the metal sheet. The metal layer can also be applied to certain parts of the metal sheet only. The metal oxide layer is formed by oxidation of the metal layer applied to the metal sheet.
The metal oxide layer can also be applied directly to the metal sheet, so that the metal sheet has catalytic activity as soon as the metal oxide layer has been applied to the metal sheet.
According to a further advantageous refinement of the structure, the metal of the metal oxide layer can have a higher oxidation potential than constituents of the metal sheet. The result achieved by this is that primarily a catalytically active metal oxide layer is formed. The metal preferably does not form a stable passivation layer at the temperatures prevailing during use of the catalytic structure.
According to another advantageous refinement of the structure, the metal of the metal oxide, or the metal oxide itself, have at most a slight capacity for diffusing into the metal sheet. This prevents the metal, before the metal oxide is formed, or the metal oxide from excessively diffusing into the metal sheet, so that a catalytically active metal oxide layer remains on the metal sheet. In particular, the at most slight diffusion must also be ensured at elevated temperatures.
In order to ensure that the activity of the catalytic metal oxide layer is also unaffected, or affected only slightly, by the diffusion of material components of the metal sheet into the metal oxide layer, the structure can be configured such that the diffusion of all material components of the metal sheet into the metal of the metal oxide layer, or into the metal oxide layer, is at most slight. This prevents the metal oxide layer from becoming enriched with constituents of the metal sheet.
According to a particularly advantageous refinement of the structure according to the invention, the metal oxide is a titanium oxide, which has particularly high catalytic activity even compared with the other fourth period oxides according to the invention. The metal sheet itself is preferably formed from a material which contains titanium as an alloy constituent. The material of the metal layer can also contain iron, chromium and nickel as alloy components.
The metal oxide is preferably formed by a zinc oxide which is formed on a corrosion-resistant base material. The catalytic activity of zinc oxide is comparatively high for oxidation reactions. It is higher than that of iron oxide or chromium oxide. The layer of zinc as the metal layer may be applied by dipping the metal sheet in liquid zinc, or by spraying zinc (in liquid form or as dust) or alternatively by electrolytic deposition from a zinc salt solution.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a catalytically active structure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.